Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device that can be positioned partially or fully behind the panel so that the touch-sensitive surface can substantially cover the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of drive and sense lines, with sensors or pixels present where the drive and sense lines cross over each other while being separated by a dielectric material to form a capacitive sensing node. In order to scan a touch sensor panel and compute an image of touch, various phases of selected frequencies can be used to simultaneously stimulate the drive lines of the touch sensor panel, and one or more mixers can be configured to demodulate the signals received from the sense lines using the selected frequencies. The demodulated signals from the mixers can be used in calculations to determine an image of touch for the touch sensor panel at each frequency.
A concern with touch devices is the amount of power they consume when scanning the touch sensor panel. The high power consumption problem can be particularly important for hand-held or battery powered devices, as part of the power consumed by device can be due to repeated charging of the stray capacitances of the drive electrodes in the multi-touch panel.